Hydrogen receives attention as the main future energy source capable of replacing existing energy since hydrogen is light-weight, abundant and environmentally superior. Normally, H2 is prepared using a reforming and/or a water-gas shift reaction of hydrocarbon fuels, and is used as a raw material of chemical synthesis, reducing gas in a semiconductor manufacturing process, and a fuel of fuel cells after being separated from other reformates or reactant gas. In other words, hydrogen obtained from hydrogen-including resources such as water, natural gas, coals or biomass includes impurities, and thereby needs to be separated and purified prior to use.
As methods for preparing and purifying hydrogen, a large number of technologies such as a cryogenic separation method, an adsorption method or a hydrogen separation method using a separation membrane have been proposed. Of these, a hydrogen separation method using a separation membrane has been widely used since the method has advantages in that the method saves more energy compared to other hydrogen separation methods, the operation is simple, the devices used in the method can be miniaturized, and the like.
A separation membrane used for purifying ultrapure hydrogen is a foil-formed palladium or a palladium alloy membrane, and it causes low hydrogen permeability due to the large thickness, therefore, in order to improve this problem, studies are under progress mainly focusing on coating a thin palladium or a palladium alloy membrane on a porous support, thereby improving the selective permeability of the membrane.
A palladium-based metal separation membrane has high hydrogen permeability and excellent hydrogen separability. In addition, a hydrogen separation membrane using a palladium-based metal separation membrane may prepare pure hydrogen useful for fuel cells or other hydrogen-consuming processes, and may be variously applied such as being used in a hydrogenation or a dehydrogenation reaction process in order to improve the quantity of target products.
In the process of hydrogen being separated from a palladium-based metal separation membrane, hydrogen molecules (H2) are diffused to the surface of a Pd metal membrane, and the hydrogen molecules are adsorbed to the surface of the Pd metal membrane, and the adsorbed hydrogen molecules are dissociated, and after the dissociated hydrogen atoms (H) are diffused through the Pd metal membrane lattice, hydrogen molecules are regenerated, and when the hydrogen molecules are regenerated, the hydrogen molecules are desorbed from the surface of the Pd metal membrane. Hydrogen is separated through this process. The operating temperature of a hydrogen separation membrane normally ranges from 300 to 500° C.
In a palladium-based metal separation membrane, the amount of hydrogen permeation is mainly determined by hydrogen partial pressure P1 on the raw material side, and hydrogen partial pressure P2 on the purified side, a membrane thickness t of the palladium-based metal separation membrane, and a membrane area of this metal separation membrane. In other words, the amount of hydrogen permeation per unit area Q has a relation of Q=A·t−1·(√{square root over (P1)}−√{square root over (P2)}). In the equation, A is different depending on the types of a metal membrane, operating conditions or the like.
As seen from the above equation, in order to improve the performances of the hydrogen permeation membrane, that is, to improve the amount of hydrogen permeation per unit area, I. developing an alloy having a large integer A that is different depending on the types of an alloy, II. thinning the hydrogen permeation membrane, or III. enlarging the partial pressure difference of hydrogen, may be considered. In a palladium alloy based-hydrogen permeation membrane, a method of improving hydrogen permeability by thinning the membrane is usually considered. However, when the membrane becomes thin, the mechanical strength becomes weak. The amount of hydrogen permeation is affected by partial pressure differences of hydrogen, therefore, thinning and strengthening are both required. Accordingly, a palladium alloy having a thin membrane is used in combination with a porous support in order to supplement the mechanical strength. However, methods for preparing a hydrogen separation membrane, which coat a palladium alloy on existing porous supports, may prepare a thin palladium alloy membrane, however, there are problems in that pin holes are easily formed, and hydrogen permeability is reduced by palladium or a palladium alloy being plated inside a porous support. As one example, when a palladium or palladium alloy membrane is formed by supplying a plating solution on a porous support, the plating solution penetrates through the lower surface of a metal support, or the plating solution introduced to the upper surface of the support readily penetrates inside since the lower surface of the porous support is open, and therefore, there is a problem of the palladium or palladium alloy layer growing even to the inside of the support.
In addition, when a metal separation membrane is directly formed on the surface of a porous support made of metals among porous supports, hydrogen permeability may be reduced due to mutual diffusion, therefore, a shielding layer made of ceramic is placed between the porous support and the metal separation membrane. As methods forming such a shielding layer, a sol-gel method or a sputtering method is used.
Meanwhile, as existing documents that provide technological descriptions relating to a palladium or palladium alloy composite membrane for hydrogen gas separation and a preparation method thereof, Korean Patent Number 10-0312069 may be included as an example. The above patent provides a method for forming a palladium alloy composite membrane for hydrogen gas separation in which a silica thin film layer is formed between a porous metal support and a plating layer of the palladium alloy, wherein the method includes a process of forming the plating layer of a palladium alloy on the other side of the support while depressurizing one side of the support in which the silica thin film layer is formed.
In the relating document described above, a composite membrane for hydrogen gas separation having thermal and chemical stability between metals on a porous support and at the same time, having a high hydrogen permeability coefficient and separation efficiency is provided, and a preparation method thereof, is provided, however, measures capable of stably and simply preparing a composite membrane for hydrogen gas separation with a uniform thickness on a porous support are not specifically described.